This invention relates to an improved method of making amorphous semiconductor alloys and devices having a substantially increased reaction gas conversion efficiency and a substantially increased deposition rate. The invention more particularly relates to a method of making such alloys and devices by plasma deposition from reaction gases wherein the plasmas are excited by microwave energy at low pressures. The invention has its most important application in making photoresponsive alloys and devices for various photoresponsive applications including photoreceptive devices such as solar cells of a p-i-n, p-n, Schottky or MIS (metal-insulator-semiconductor) type; photoconducting medium such as utilized in xerography; photodetecting devices and photodiodes including large area photodiode arrays.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 percent) crystalline solar cells for space applications. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction crystals involve extremely complex, time-consuming, and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impossible economic barrier to the large-scale use of the crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involves the addition of grain boundaries and other problem defects.
In summary, crystalline silicon devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. Devices based upon amorphous silicon can eliminate these crystalline silicon disadvantages. Amorphous silicon has an optical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous silicon can be made faster, easier and in larger areas than can crystalline silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and of substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein a gas of silane (SiH.sub.4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree.-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, a gas of phosphine (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.2 H.sub.6) for p-type conduction was premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited included supposedly substitutional phosphorus or boron dopant and was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous material approximate more nearly those of the corresponding crystalline material.
The incorporation of hydrogen in the above method not only has limitations based upon the fixed ratio of hydrogen to silicon in silane, but, more importantly, various Si:H bonding configurations introduce new antibonding states which can have deleterious consequences in these materials. Therefore, there are basic limitations in reducing the density of localized states in these materials which are particularly harmful in terms of effective p as well as n doping. The resulting density of states of the silane deposited materials leads to a narrow depletion width, which in turn limits the efficiencies of solar cells and other devices whose operation depends on the drift of free carriers. The method of making these materials by the use of only silicon and hydrogen also results in a high density of surface states which affects all the above parameters.
After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter depositing of amorphous silicon films in the atmosphere of a mixture of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as an altering agent which bonded in such a way as to reduce the localized states in the energy gap. However, the degree to which the localized states in the energy gap were reduced in the sputter deposition process was much less than that achieved by the silane deposition process described above. The above-described p and n dopant gases also were introduced in the sputtering process to produce p and n doped materials. These materials had a lower doping efficiency than the materials produced in the glow discharge process. Neither process produced efficient p-doped materials with sufficiently high acceptor concentrations for producing commercial p-n or p-i-n junction devices. The n-doping efficiency was below desirable acceptable commercial levels and the p-doping was particularly undesirable since it reduced the width of the band gap and increased the number of localized states in the band gap.
Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high-quality electronic properties have been prepared by glow discharge as fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980, and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, which are incorporated herein by reference, fluorine is introduced into the amorphous silicon semiconductor to substantially reduce the density of localized states therein. Activated fluorine especially readily diffuses into and bonds to the amorphous silicon in the amorphous body to substantially decrease the density of localized defect states therein, because the small size of the fluorine atoms enables them to be readily introduced into the amorphous body. The fluorine bonds to the dangling bonds of the silicon and forms what is believed to be a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen because of its exceedingly small size, high reactivity, specificity in chemical bonding, and highest electronegativity. Hence, fluorine is qualitatively different from other halogens and so is considered a super-halogen.
As an example, compensation may be achieved with fluorine alone or in combination with hydrogen with the addition of these element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages so as to form a silicon-hydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or greater. It is believed that the new alloy so formed has a lower density of defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that fluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the density of defect states which hydrogen contributes while acting as a density of states reducing element. The ionic role that fluorine plays in such an alloy is believed to be an important factor in terms of the nearest neighbor relationships.
Amorphous semiconductor alloys made by the processes hereinabove described have demonstrated photoresponsive characteristics ideally suited for photovoltaic applications. These prior art processes, however, have suffered from relatively slow deposition rates and low utilization of the reaction gas feed stock which are important considerations from the standpoint of making photovoltaic and particularly xerographic devices from these materials on a commercial basis. In addition, these prior art processes, when practiced utilizing high RF power densities to enhance deposition rates result in the production of films with poor electrical properties, increased densities of defect states and in the production of powder in the reaction vessel due to gas phase nucleation processes.
Many techniques of exciting conventional glow discharge plasmas have been investigated. These have included direct current (DC) and alternating current (AC) techniques. Various AC frequencies have been utilized, such as audio, radio frequency (RF) and a microwave frequency of 2.56 GHz. It is known that the optimum deposition power and pressure are defined by the minimum of the Paschen curve. The Paschen curve defines the voltage (V) needed to sustain the glow discharge plasma at each pressure (P) in a range of pressures, between electrodes separated by a distance (D). In a typically sized, conventional RF glow discharge system, the minimum in the Paschen curve occurs at a few hundred mTorr.
The problem addressed by the present invention is how to achieve a high reaction gas conversion efficiency and a high deposition rate without substantially degrading the properties of the resulting alloys. It has previously been discovered that increasing the applied RF power increases the gas utilization efficiency and the deposition rate. However, simply increasing the RF power to achieve deposition rates approximately greater than 10 .ANG./sec. leads to the production of amorphous semiconductor films of decreasing electronic quality and can result in films which include polymeric material and/or the production of powder. The increased deposition rate with increased RF power is a result of an increase in the concentration of excited species resulting principally from collisions between electrons and feedstock molecules. However, the collision rate between excited species and more importantly between excited species and feedstock molecules is also increased. This results in the formation of polymer chains. These chains are either incorporated in the growing amorphous semiconductor film degrading its electronic quality or condensed in the gas phase to produce powder particles. To reduce the number of undesirable collisions one can reduce the operating pressure, but this moves the deposition process off the minimum of the Paschen curve and substantially higher RF power is required to achieve the same degree of plasma excitation. The physical reason for this phenomenon is that, as pressure is reduced, many electrons that would have been able to collisionally excite feedstock molecules at higher gas pressures now impinge on the substrate or system walls without suffering collisions.
Attempts have also been made to increase the gas utilization efficiency in RF glow discharge plasmas by high power deposition of a dilute mixture of silane (SiH.sub.4) in an inert carrier gas such as argon. However, this is known to result in undesirable film growth conditions giving rise to columnar morphology as reported by Knights, Journal of Non-Crystalline Solids, Vol. 35 and 36, p. 159 (1980).
The one group which has reported glow discharge amorphous silicon alloy deposition utilizing microwave energy at 2.54 GHz treated the microwave energy as just another source of plasma excitation by performing the deposition in a plasma operating at pressures typical of conventional RF or DC glow discharge processes. C. Mailhiot et al. in the Journal of Non-Crystalline Solids, Vol. 35 and 36, p. 207-212 (1980) describe films deposited at 0.17 Torr to 0.30 Torr at deposition rates of between 23 and 34 .ANG./sec. They report that their films, which are of poor electrical quality, show clear indication of non-homogeneous structure. Thus, Mailhiot et al. failed to discover the present invention which is based on the recognition that for a given deposition system the minimum in the Paschen curve shifts to lower pressure values with increasing frequency. Therefore, the use of high frequency microwave energy in a glow discharge deposition system allows one to operate at much lower pressure and consequently to achieve a higher concentration of excited species and thus higher deposition rate and gas utilization efficiency without production of powder or inclusion of polymeric species in the amorphous semiconductor film. The shift in the minimum of the Paschen curve is believed to occur because, for a given gas pressure at the higher excitation frequency, the rapid reversals of the applied electric field allow the electrons in the plasma to collide with more feedstock molecules in the plasma excitation region before they encounter the walls of the system. Thus, the present invention provides both a substantially increased deposition rate of 100 .ANG./sec. or above and a feedstock conversion efficiency approaching 100% while still allowing the production of high electrical quality amorphous semiconductor films. This contrasts both with the conventional RF (eg., 13.56 MHz, 0.2 to 0.5 Torr) glow discharge deposition process which produces high quality films at deposition rates of approximately 10 .ANG./sec. and feedstock utilization of approximately 10% and with the Mailhiot microwave process (2.54 GHz, 0.2 Torr to 0.3 Torr) which produced poor quality films at 20 to 30 .ANG./sec. deposition rates.
Applicants herein have discovered a new and improved process for making amorphous semiconductor alloys and devices. The inventive process herein provides substantially increased deposition rates and reaction gas feed stock utilization. Further, the process of the present invention results in depositions from plasmas operated at lower pressure and consequently capable of powderless depositions of a semiconductor film with only small amounts of incorporated polymeric material. Still further, the process of the present invention results in the formation of reactive species not previously obtainable in sufficiently large concentrations with prior art processes. As a result, new amorphous semiconductor alloys can be produced having substantially different material properties than previously obtainable. All of the above results, by virtue of the present invention, in amorphous semiconductor alloys and devices made therefrom having improved photoresponsive characteristics while being made at substantially increased rates.
As disclosed in the aforementioned referenced U.S. Pat. No. 4,217,374, new and improved amorphous semiconductor alloys and devices can be made which are stable and composed of chemical configurations which are determined by basic bonding considerations. One of these considerations is that the material is as tetrahedrally bonded as possible to permit minimal distortion of the material without long-range order. Fluorine, for example, when incorporated into these alloy materials, performs the function of promoting tetrahedral bonding configurations. Amorphous semiconductor materials having such tetrahedral structures exhibit low densities of dangling bonds making the materials suitable for photovoltaic applications.
In accordance with one preferred embodiment, atomic fluorine and/or hydrogen are generated separately as free radicals and reacted with semiconductor free radicals generated within a plasma sustained by microwave energy. As a result, all of the advantages of separate free radical formation are obtained along with all of the advantages of microwave plasma deposition.
In making conventional xerographic devices, materials such as various amorphous chalcogenide alloys including Se.sub.92 Te.sub.8 ; As.sub.2 Se.sub.3 ; or organic alloys such as TNF-PVK have been utilized. These materials are soft, easily damaged in use, and toxic. Amorphous silicon alloys are much more desirable for this application since they are hard, non-toxic, and capable of being formulated with a wide range of spectral response characteristics. Low deposition rates and low gas utilization efficiencies have, however, hindered their commercial application. One preferred embodiment of the invention makes possible the formation of electrophotographic members by the high gas utilization efficiency and the high deposition rate achieved by the present invention.
In making a commercial photovoltaic device, it is necessary to provide environmental encapsulation of the devices to prevent undesirable chemical reactions within the device materials due to exposure to chemical species contained in the environment. For example, oxidation of contact materials must be prevented. Customarily, relatively heavy and thick materials such as glass plates or various organic polymer layers or plastic materials have been proposed to provide such protection. In accordance with a further embodiment of the present invention, such protection is provided by microwave glow discharge deposition of appropriately hard, chemically inert wide bandgap semiconductor material, deposited at high deposition rates. The microwave deposited material not only provides the required encapsulation, but is additionally light in weight and can be easily incorporated in a manner compatible with the formation of the photovoltaic materials of the devices.